(1) Field of the Invention
The present invention relates to a collimator array, which is one of optical parts to be applied to an optical communication system and a method for manufacturing the same.
(2) Description of the Related Art
For example, a collimator is an optical part for space-communicating collimated plural optical beams in parallel, which is applied to a spatial optical system type module such as an optical switch or the like. Normally, the collimator array is constructed by a plurality of lenses and a plurality of optical fibers. As a conventional collimator array, there is a configuration that a lens array and a fiber array are combined.
In this case, for example, the lens array may be configured by etching on a substrate such as glass, silicon or the like, a platy lens array may be formed by means of molding or the like, or the lens array may be configured by laying lenses in a V-shaped grove provided on a substrate. In addition, with respect to a fiber array, there are methods for manufacturing it by arranging fibers in the V-shaped groove and by using a fiber insertion hole that is processed at a high degree of accuracy.
These methods have an advantage that alignment can be made collectively by individually configuring the lens array and the fiber array, respectively and then, adhering the both by an adhesion bond or the like. However, according to this method, a deviation is generated at any cost at a center of each lens and each fiber due to each limit on arrangement accuracy of the lens array and the fiber array, and this results in occurrence of variation in an output angle of a beam.
A condition necessary for a collimator array is that there is little variation in a parallel degree and a beam diameter between the beams to be propagated in an array. In this case, the parallel degree between the beams is decided by the arrangement position accuracy of each fiber and each lens. A beam output angle θ is represented as follows:θ=arctan(x/f)
where, x represents a difference between a fiber center position and a lens center position, and f represents a lens focal distance. Accordingly, if the arrangement accuracy of the fiber or the lens is not good, variation may occur in the beam output angle.
According to the above-described method, even if a method such as etching whereby the arrangement of the lens can be made with a high degree of accuracy is employed, a sub-micron order is a limit on the lens arrangement accuracy and even if the V-shaped groove is used, a sub-micron order is also a limit on the fiber arrangement accuracy. Further, since a core position of each fiber is decentered, variation in a beam output angle is large and about ±0.01° to ±0.1° of variation occurs.
In order to realize a higher degree of accuracy of the beam output angle, it is necessary that any one of the lens array and the fiber array is used as an individual part and alignment is carried out in accordance with arrangement variation in the one while observing an optical beam. This example is shown in FIG. 15(A) and FIG. 15(B). The configuration shown in these FIG. 15(A) and FIG. 15(B) are proposed by a patent document 1 to be described later, and in this configuration, positions (center axes) of a plurality of fibers 11 (a fiber part) with ferrules 12 is actively adjusted to a rear surface 10c of a platy micro lens array 10 (a substrate 10a) so as to coincide with an optical axis 19 of a lens 13 and fix the fibers 11 by an adhesion bond 14, respectively.
In the meantime, in these FIG. 15(A) and FIG. 15(B), a reference numeral 11a denotes an output end face of the fiber 11, a reference numeral 12a denotes an end face of a ferrule 12, a reference numeral 16 denotes a space (a gap) between the ferrule 12 and a rear surface 10c of the lens array 10, a reference numeral 18 denotes a focal position of the lens 13, a reference numeral 20 denotes a collimated light by the lens 13, and a reference numeral f denotes a focal distance of the lens 13, respectively. The fibers 11 are adhered by the adhesion bond 14 so that the focal position 18 of the lens 13 is located at a center of a core of the fiber 11 that is exposed on the end face 12a of the ferrule 12.
According to a method (structure) of this patent document 1, it is possible to coincide the center axis of the lens with the center axis of the fiber, relative variation in the output angle of each beam can be prevented and variation in a beam diameter can be also decreased by adjusting a thickness 14 of the adhesion bond in accordance with variation in the focal distance of the lens and further adjusting a distance between the fiber and the lens.
In addition, as a method for fixing the lens array and the fiber array, there is a method by fusion-connecting other than the adhesion. For example, according to a patent document 2 to be described later, a shape of a lens is limited to an ellipse SIL (solid immersion lens), however, there is a description with respect to the fiber fusion-connecting to the lens array. This fiber fusion-connecting is shown in FIG. 16(A), FIG. 16(B), and FIG. 16(C). In the meantime, all of these FIGS. 16(A) to 16(C) are pattern plain views of a collimator array and they correspond to FIGS. 6A to 6C of the patent document 2.
In these FIG. 16(A) to FIG. 16(C), a reference numeral 200 denotes a lens array substrate; a reference numeral 201 denotes a front surface (a substrate front surface) of the substrate 200; a reference numeral 270 denotes a rear surface (a substrate rear surface) of the substrate 200; reference numerals 202, 204, and 266 denote attachment positions of fibers (wave guide paths) 220 and 222, respectively; reference numerals 210, 212 denote a lens (SIL: Solid Immersion Lens) configuring a lens array 213, respectively; reference numerals 214, 216 denote focal positions of lenses 210, 212, respectively; and reference numerals 234, 236, 260, and 262 denote projecting parts, respectively.
In addition, an arrow 221 denotes a cross section of the fiber 220 at that position; an arrow 238 denotes a cross section of the projecting part 234 at that position; arrows 264 and 268 denote cross sections of the projecting part 260 at respective positions. In FIG. 16(B), the cross sectional shape of the projecting part 234 coincides with that of the fiber 220, and in FIG. 16(C), the cross sectional shape of the fiber attachment position 260 of the projecting part 268 coincides with that of the fiber 220.
In the meantime, in FIG. 16(A), the lenses 210 and 212 are formed on one surface (the substrate front surface) 201 of the substrate 200, and the fibers 220 and 222 are fusion-connected on the positions corresponding to the lenses 210 and 212 of the other surface (the substrate rear surface) 270 of the substrate 200, respectively. This structure is nearly the same as the structure described in the patent document 1 shown in FIG. 16 except for a point that the ferrule 12 is not attached to the fibers 220 and 222.
On the other hand, in FIG. 16(B) and FIG. 16(C), the projecting parts 234 and 236 (260, 262) are formed at the positions corresponding to the positions where the lenses 210 and 212 of the substrate rear surface 270 are formed, and the fibers 220 and 222 are fusion-connected to these projecting parts 234 and 236 (260, 262), respectively. Thus, by providing the projecting parts 234 and 236 (260, 262), as shown in FIG. 16(B) (FIG. 16(C)), the fusion-connecting by arc discharge and CO2 laser irradiation or the like becomes possible as compared to a case that the fibers 220 and 222 are directly fusion-connected to the substrate rear surface 270.
In addition, by coinciding appearances (cross sectional shapes) of the fiber connected faces of the projecting parts 234 and 236 or 260 and 262 with the fiber appearance (the cross sectional shape), there is an advantage that alignment of the center of the lens and the center of the fiber can be obtained easily. Actually, if the projecting parts 234 and 236 or 260 and 262 as shown in these FIGS. 16(B) and 16(C) are used, upon fusion-connecting the fibers, a force may act so that the appearances of the both coincide with each other by the surface tension generated at a connected portion that is fusion-connected upon fusion-connecting the fibers, and this makes it possible to fix the fibers in such a manner that the appearances of the projecting parts 234 and 236 or 260 and 262 coincide with the appearances of the fibers.
In the meantime, as the art related to an apparatus for fusion-connecting the fiber, for example, the art described in the following non-patent document 1 is conceivable. [patent document 1] a specification of U.S. Pat. No. 6,587,618 [patent document 2] a specification of U.S. Pat. No. 6,643,068 [non-patent document 1] Akio Tanabe and other nine persons, “Development of Core Direct Sight Type Fusion Apparatus S175”, Furukawa Denko Times, July 1999, No. 104, pp. 69-74
However, at first, according to the art in the above-described patent document 1, adhesion is used to connect the fiber to the substrate, so that fixing strength is weak. In addition, since a direction of a lens focal distance is adjusted by changing the thickness of the adhesion bond, a connection layer becomes thick and thus the art lacks reliability. Further, the connected position may be deviated by curing and contraction of the adhesion bond upon bonding.
On the contrary, according to the art in the above-described patent document 2, since the fiber is connected to the substrate by fusion-connecting, the fixing strength is improved as compared to the art according to the patent document 1, however according to the method of directly fusion-connecting the fiber on the substrate as described with reference to FIG. 16(A), it is difficult to carry out arc discharge or laser irradiation to individual connected portions at a pinpoint and the fusion-connecting operation is very difficult because the unnecessary substrate parts are fused by heat conduction to the periphery. In addition, it is also difficult to adjust the direction of the lens focal distance.
On the other hand, according to the methods above-described with reference to FIG. 16(B) and FIG. 16(C), the fusion-connecting operation is made simple, however, the appearances of the fiber connected faces of the projecting parts 234, 236 or 260, 262 formed on the substrate rear surface 270 and the fiber appearances are fixed so that they coincide with each other by the surface tension. This phenomenon is also described in the above-described non-patent document 1, for example, with respect to a case of fusion-connecting the fibers of the same appearance with each other. Due to the phenomenon as same as this, a deviation may occur between the lens center axis of the substrate front surface 201 and the center axes of the projecting parts 234, 236 or 260, 262 of the substrate rear surface 270. In addition, if the fiber core is decentered, there may be variation also in the beam output angle.
For example, as shown in a patterned manner in FIG. 17(A), the center axis of the lens 210 (212) on the substrate front surface 201 and the center axis of the projecting part 234 (236, 260, 262) on the substrate rear surface 270 formed on the position corresponding to this lens 210 (212) are deviated, and as shown in a patterned manner in FIG. 17(B), it is assumed that a fiber core 240 is decenterd with respect to a fiber clad 241. With respect to the center axis and the decentering, a deviation to a lens center 250 is not more than 0.5 to 1 μm, however, in these FIG. 17(A) and FIG. 17(B), the deviation is overly illustrated for the sake of convenience of explanation.
In addition, if the fiber 220 (222) is fusion-connected to the projecting parts 234 (236, 260, 262) under such a state, as a result of a fact that the appearances of the fiber connected faces of the projecting parts 234, 236 or 260, 262 and the fiber appearances are fixed so that they coincide with each other more by the surface tension, the fiber core 240 is largely deviated from the lens center 250, and as shown in a patterned manner in FIG. 18, there is variation in the beam output angle. In addition, if there is variation in the thickness of the projecting part 234 (236, 260, 262) and also in the lens focal distance, it leads to variation in the collimated state (the beam diameter) of the beam.